The performance of hard disk drives is subject to continuous improvement to enable the design of progressively larger storage capacities and more rapid data processing. This must be accomplished within the confines of fixed form factors. These form factors have become smaller with the sequence of 51/4 inch drives, 31/2 inch drives and finally to the presently available PCMCIA drives that include a standard with an overall device height of 5 mm. With increasing track density, the advent of portable disk drives and continuing cost reduction efforts, the distance to move from head-to-head is larger than the distance from cylinder-to-cylinder on the same data surface. This is especially true for portable drives, where it is desirable to allow for head-to-head mechanical tolerances of up to 2000 microinches, such as may occur after a tolerable shock event such as an "acceptable" drop of the device. In general, the present invention is progressively more useful as track pitch increases beyond 5000 tpi with head-to-head distances of 175 microinches.
The sequence of data tracks recorded on a disk drive have typically been arranged in order of increasing cylinder, and tracks assigned to sequential heads on each cylinder. This track layout requires a head switch for each track switch, meaning that the next read/write head is selected, and position offset movement performed if necessary. The amount of time required to complete a head switch contributes significantly to the amount of format skew used. Larger skew time allows for increased track switch time, but reduces the average transfer rate on sequential or long random operations.
In disk drives with multiple data disks, the sequence of data tracks recorded on a disk drive has typically been arranged in order of increasing head number on each cylinder, returning to head 0 at each cylinder boundary. Given this arrangement, the head-to-head switch distances are a fraction of the total mechanical offsets in the head/disk stack, but the head switch and cylinder increment from the last head on cylinder "n" to the first head on cylinder "n+1" may require movement of several cylinders in distance. The amount of time required to complete a head switch or cylinder increment is accounted for in the data format with track skew and cylinder skew. These are rotational offsets which allow the actuator to be repositioned when the read write operation spans a track boundary. Thus if the head skew and cylinder skew times require 1/16 revolution and 1/8 revolution, then it will take 8+(7.times.1/16)+(1.times.1/8)=8.5625 revolutions to read or write an entire cylinder of an 8 surface disk drive. To obtain enhanced performance, it is desirable to minimize the head-to-head and cylinder increment distances to maximize the average data transfer rate.
In a typical single disk drive, using a conventional track format, each track switch requires a head switch operation, with head 1 to head 0 also incrementing the cylinder. An alternative format is also currently in use today in some drives, is referred to as sequential track format, wherein sequential tracks are accessed across the data band. This reduces the need for head switches, but uses only one surface until half the disk storage capacity is consumed. A consequence of this format is the requirement for more lengthy accesses. In devices using multiple disks, it is common to access successive tracks in the same cylinder, with the same sequence of heads being used in each cylinder. By skewing each track within the cylinder it is possible to optimize track to track seek times. Such a method is shown and described in U.S. Pat. No. 5,193,036. However, when track misregistration occurs the benefits of skewing the tracks within the cylinder can be defeated by extended cylinder increment times.